As the treatment for improving the adhesion between an organic film and a metal material such as steel sheet, zinc-based plated steel sheet, zinc-alloy sheet, tin-based plated steel sheet and aluminum alloy sheet, there have heretofore been known chromate treatment, phosphate treatment, silane coupling treatment, etc. Among these, the chromate treatment has been broadly utilized in the fields of home electrical appliances, building materials, vehicles, metal containers, etc., due to its superior corrosion resistance and adhesion. However, there has been pointed out the possibility of the toxic substance of hexavalent chrome contaminating the soil, etc., by the leaching thereof into the soil at the time of the disposal of the chromate-treated products. Accordingly, the industries mainly in Europe, are ready to eliminate the chromate treatment at the present stage.
In the field of metal materials to be used for containers, a certain type of chromate treatment method is being utilized, such that a tin-plated steel sheet is treated by cathodic electrolysis in an aqueous solution of sodium bichromate, or a steel sheet is treated by cathodic electrolysis in an aqueous solution of fluorine-containing anhydrous chromic acid, so as not to leave hexavalent chrome in the resultant film. However, even in the case of the chromate treatment of a type where the treated layer does not include hexavalent chrome, the treatment solution to be used therefor contains the hexavalent chrome, and accordingly, the hexavalent chrome has to be rendered harmless for the treatment or disposal of the wastewater and gas emissions. For this reason, from the viewpoint of the environmental load, a surface treatment is desirable such that the treatment solution does not include hexavalent chrome either.
From this viewpoint, in order to make a treatment solution hexavalent chromium-free, attempts to eliminate chromium have come to attract attention, and such chromium-free attempts include investigation on the removal of chromium from a coating film or plating per se, a coating film or alternative plating which is alternative to chromium or chromium plating.
Further, with respect to fluorine, boron, nitrate nitrogen, etc., are also not preferable from the viewpoint of the environmental load. In the future, the industries will be encountered with toughened emission standards therefor. Therefore, the treatment solutions for metal materials to be used for containers may preferably be those which do not contain the substances as described above.
Therefore, as one measure for reducing the environmental load, there is elimination of the use of chrome. Patent Document 1 discloses an example of the method of surface-treating a tin-plated steel can superior in corrosion resistance and coating adhesion, wherein a container material is chrome-free surface-treated by providing, on a tin-plated surface of a tin-plated steel sheet, an organic-inorganic composite coating comprising an organic compound main comprising carbon and an inorganic phosphorus compound. Patent Document 2 discloses, as a surface treatment solution for an aluminum can or tin-plated DI (drawing and ironing) can prior to the coating and printing thereof, an example of the surface treatment solution for DI can, which contains at least one kind of phosphoric acid ions and a zirconium compound and titanium compound, and contains an oxidizing agent and at least one kind of fluoric acid and a fluoride.
Conventionally, the metal containers to be used for beverage can and food can applications have generally been treated so as to bake the coatings at the inside and outside surfaces of the cans, after the manufacturing of the cans. In recent years, as the metal materials to be used for beverage cans or food cans, there have been increasingly used a steel sheet with a film which has been hot-laminated on the steel sheet in advance, and a pre-coated steel sheet comprising a steel sheet which has been subjected to a coating treatment including printing and baking, in advance.
However, in the can manufacturing using DI or DRD (drawing and redrawing), an ironing force acts on the can wall, so in a case where a can is manufactured by using a laminated steel sheet or coating pre-coated steel sheet type of metal material for container, if the adhesive strength between the resin coating and the steel sheet is not sufficient, there is caused a problem such that the resin coating will easily be peeled off. Further, in the sterilization (i.e., retort treatment) which is to be performed after filling of the can with a content, water in the content sometimes permeates the resin coating under the high temperature and high pressure conditions, and the adhesion is liable to be decreased. Accordingly, even in the development of the chromium-free type steel sheet for container material, it is necessary to attain an excellent adhesion between the resin coating and the steel sheet.
With respect to such a requirement for the container materials, as disclosed in Patent Document 3, the present inventors have developed a chromium-free steel sheet for a container material having an excellent adhesion in which a zirconium compound-containing coating film is formed on a tin-plated steel sheet, and have developed a steel sheet for container material having an adhesion which is equal to or greater than that of the conventional chromate treatment. However, the invention disclosed in Patent Document 3 had a drawback such that, in the electrolytic treatment therefor, it is necessary to finely regulate the electrolytic conditions during the treatment in order to maintain the coating amount in an appropriate range.